<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
    @media only screen and (max-width:750px){
     .head{
         background-color: #f00;
     }
    }
    @media only screen and (min-width:750px){
     .head{
         background-color: #ff0;
     }
    }
    </style>
</head>

<body>
    <header class="head">我是头部内容</header>
</body>
</html>